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The electronic absorption spectra of Cr4+ in Rb2CrF6 and Cr2+ in KCrF3 have been studied using ab initio molecular orbital methods. Near-degeneracy effects within the dn (n=2,4) manifold are treated using the complete active space (CAS) self-consistent field method, while dynamical correlation is dealt with using both the averaged coupled-pair functional (ACPF) method or second-order perturbation theory (CASPT2). The ground state electronic structure of the CrFn-6 (n=2,3,4) clusters in the ionic crystals Rb2CrF6, K2NaCrF6, KCrF3 was analyzed first using Bader’s theory of atoms in molecules. The topological analysis of the charge density is consistent with an ionic picture of the chemical bond in all three compounds, although the contribution of covalent effects clearly increases in the series CrF4-6

The ground and ligand field excited states of CrF3-6 have been studied, using different Gaussian basis sets of atomic natural orbitals. Each state was first optimized separately in a complete active space self-consistent field (CASSCF) calculation, including three active electrons in the 2t2g and 4eg shells. Further correlation was then added by using either a singles and doubles configuration interaction approach (SDCI) or by the average coupled pair functional method (ACPF) with the CASSCF configuration space as the reference space. Thereby the number of correlated electrons was raised up to 15. It is shown that the quartet–quartet transitions, corresponding to a 2t2g→4eg excitation, are described already very accurately at the CASSCF level. Further improvement of the 4A2g→4T2g transition was obtained by extending the CI treatment so as to include the F 2p electrons from the 1t2g,3eg, and finally also from the 6a1g shell. For the intraconfigurational t32g quartet–doublet transitions on the other hand, the best results were obtained by an 11 electron CI treatment, including the Cr(III) 3s and 3p electrons. [ABSTRACT FROM AUTHOR]